On the influence of binder content and binder composition on the mechanical properties of hardmetals

Abstract

In order to achieve an understanding of damage mechanisms and the role of the binder phase in the fatigue behaviour of hardmetals, two series of hardmetals with 7, 15 and 27 wt% of Co and CoNiFe binder phases were investigated in a reversed bending stress apparatus. Microstructural investigations were carried out using SEM and TEM. Finite element simulations based on real microstructure cut-outs were used to analyse the stress and strain states. While both series of hardmetals exhibit inert strengths which depend strongly on the binder content, their fatigue behaviour is totally different. For the WC–Co grades, no significant influence of the binder content on the fatigue behaviour was found. In contrast to this, the fatigue behaviour of the WC–CoNiFe grades showed a clear dependence on the binder content. The microstructural investigations revealed that the mechanism of stacking fault formation and phase transformation in the Co binder phase, together with a stress concentration in the binder pools are responsible for the fatigue behaviour of the WC–Co grades. In WC–CoNiFe grades, no stacking faults or transformed zones were observed. In addition, the CoNiFe binder phase reveals a very ductile flow nature, which results in large plastically deformed zones in the binder phase.